In a position measuring apparatus for measuring the relative position of two objects, such as two machine parts of a processing machine, a probe connected to a first object probes a measuring instrument or scale, which is arranged on a support member. The support member is in turn connected to a second object.
While the second object, which is typically a machine part, generally consists of gray iron or steel, aluminum is preferably used for the support member of the measuring scale, since the support member with its complicated cross-sectional profile, especially in an encapsulated positioning measuring apparatus, can be most economically produced by extrusion processes.
If the aluminum support member is fastened rigidly, such as by screws, to the machine part of gray iron or steel considerable longitudinal forces are generated in the support member which experiences temperature changes because of the very different coefficients of thermal expansion of the support member and the machine part. The presence of different thermal expansion coefficients is unavoidable during the machining process. Further, these longitudinal forces can cause indefinite displacements of the support member in the screw connections, which entails origin, or zero point, changes in the measuring scale and non-reproducible stresses in the measuring scale along with the associated measurement irregularities.
A position measuring arrangement is described in DE-OS 28 53 771, wherein one support member, in the shape of a housing for the measuring scale, and the probing unit, is connected at both ends with one machine part by means of two fastening elements. While the one end of the housing is connected directly with the first fastening or attachment element, a length compensation element for temperature compensation is arranged between the other end of the housing and the second fastening element, so that the end of the housing is supported with a translatory degree of freedom in the longitudinal direction. This relatively expensive type of attachment of the support member for the measuring scale to origin, of the measuring scale must lie at the point of attachment of the support member to the first attachment element.